The present invention is directed to an apparatus and process for developing images. More specifically, the present invention is directed to an apparatus and process for heat development of migration imaging members. One embodiment of the present invention is directed to an apparatus for heat development of a migration imaging member containing migration marking material and a softenable material capable of softening upon exposure to heat at a development temperature, which apparatus comprises a heating source, a conveyance means for conveying the migration imaging member past the heating source, a first pinch roller in contact with the conveyance means, and a second pinch roller in contact with the conveyance means, wherein the imaging member passes through a nip between the conveyance means and the first pinch roller subsequent to entering the apparatus and prior to exposure to the heating source and passes through a nip between the conveyance means and the second pinch roller subsequent to exposure to the heating source and prior to exiting the apparatus, wherein the surface temperature of the first pinch roller is maintained at a temperature at least 20.degree. C. below the development temperature of the migration imaging member during the period in which the first pinch roller contacts the migration imaging member, wherein the surface temperature of the second pinch roller is maintained at a temperature at least 20.degree. C. below the development temperature of the migration imaging member during the period in which the second pinch roller contacts the migration imaging member, and wherein the heating source is maintained at the development temperature of the migration imaging member during development.
Migration imaging members are well known, and are described in detail in, for example, U.S. Pat. No. 3,975,195 (Goffe), U.S. Pat. No. 3,909,262 (Goffe et al.), U.S. Pat. No. 4,536,457 (Tam), U.S. Pat. No. 4,536,458 (Ng), U.S. Pat. No. 4,013,462 (Goffe et al.), and "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P. S. Vincett, G. J. Kovacs, M. C. Tam, A. L. Pundsack, and P. H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986), the disclosures of each of which are totally incorporated herein by reference. Migration imaging members containing charge transport materials in the softenable layer are also known, and are disclosed, for example, in U.S. Pat. No. 4,536,457 (Tam) and U.S. Pat. No. 4,536,458 (Ng), the disclosures of each of which are totally incorporated herein by reference. A typical migration imaging member comprises a substrate, a layer of softenable material, and photosensitive marking material in the form of a fracturable layer contiguous with the upper surface of the softenable layer. The member is imaged by first electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation, such as light, to form a latent image on the member. Subsequently, the imaged member is developed by one of several methods, such as application of heat, solvent, solvent vapor, or the like, causing the marking material in the exposed areas of the member to migrate in depth through the softenable material toward the substrate.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced apart from the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the illuminating electromagnetic radiation pattern.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. For the purpose of this invention, all values of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate used in this invention.
There are various other systems for forming such images, wherein non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patents, which also discloses a variety of methods which can be used to form latent images upon migration imaging members.
The background portions of an imaged member can sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and softening the softenable layer by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image can be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the coalesced particles towards a shape of lower energy, such as a sphere.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the imaging member and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in U.S. Pat. No. 3,909,262, the disclosure of which is totally incorporated herein by reference. However, because the migration imaging mechanisms for each development method are different and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles, and the like, application of an overcoat to the softenable layer can cause changes in the delicate balance of these processes and result in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density can degraded. Recently, improvements in migration imaging members and processes for forming images on these migration imaging members have been achieved. These improved migration imaging members and processes are described in U.S. Pat. No. 4,536,458 (Ng) and U.S. Pat. No. 4,536,457 (Tam).
Methods of developing or fixing images by heat are known. For example, U.S. Pat. No. 4,435,072 (Adachi et al.) discloses an image formation apparatus having a fixing station for applying high frequency waves to fix an image on a recording medium. In operation, a latent image is formed on a photosensitive drum, and the latent image is developed with a developer. The developed image is then transferred to a recording medium and exposed to high frequency waves to affix the transferred image to the recording medium. In one embodiment, the fixing apparatus comprises one or more pairs of rollers of a high-frequency wave absorbing material. High frequency waves are applied to the image in a manner so as to reduce escape of high frequency waves from leaking; the absorbing rollers help reduce leakage and also become heated by absorption of high frequency waves, which assists in fixing the image to the recording medium.
U.S. Pat. No. 3,997,790 (Suzuki et al.) discloses an apparatus for heat-fixing a toner image onto a support sheet wherein fixing is effected through both infrared radiation and direct contact with a heated surface of fixing roller in succession. An endless belt transparent to infrared light and trained over a pair of rollers is disposed within a heat insulating casing, an upper run of the belt defining a path of movement of a toner image-bearing support sheet to be fixed. An infrared radiator is disposed beneath the upper run of belt while a reflecting plate is disposed at the opposite of the belt. A fixing roller is disposed downstream of the radiator along the path for completing the fixing.
U.S. Pat. No. 4,077,803 (Gravel) discloses a method and apparatus for uniformly charging a single layer thermoplastic recording surface either positively or negatively to a potential just below the first threshold level for exposing the thermoplastic surface to light in image configuration, and for applying a heat pulse to the thermoplastic surface for a time relatively short compared to the duration of the light exposure interval and during the exposure. The charging event is arranged so that the thermoplastic surface is raised only to a relatively low potential with respect to ground.
U.S. Pat. No. 4,161,644 (Yanagawa et al.) discloses an electric heater means for thermally fixing a toner image to a copy sheet to produce a permanent electrostatic copy of an original document. The heater means is normally energized at partial power but is switched to full power by means of microswitches at the inlet and outlet of the heater means which are actuated by the copy sheet while the copy sheet passes through the heater means. The heater means is switched to full power for a shorter length of time during a multiple copy operation than during a single copy operation.
U.S. Pat. No. 4,751,528 (Spehrley, Jr. et al.) discloses a hot melt ink jet system including a temperature controlled platen provided with a heater and a thermoelectric cooler electrically connected to a heat pump and a temperature control unit for controlling the operation of the heater and the heat pump to maintain the platen temperature at a desired level. The apparatus also includes a second thermoelectric cooler to solidify hot melt ink in a selected zone more rapidly to avoid offset by a pinch roll coming in contact with the surface of the substrate to which hot melt ink has been applied. An airtight enclosure surrounding the platen is connected to a vacuum pump and has slits adjacent to the platen to hold the substrate in thermal contact with the platen.
Although known apparatuses and processes are suitable for their intended purposes, a need remains for apparatuses suitable for heat development of migration imaging members. In addition, a need remains for heat developer apparatuses that can develop migration imaging members without damaging them in such a manner as to impair image quality. Further, there is a need for heat developer apparatuses that do not cause dimensional distortion to migration imaging members during development, which could result in difficulties such as color misregistration when the migration imaging members are used to prepare color separation xeroprinting masters for color xeroprinting. A need also exists for heat development apparatuses that do not contact migration imaging members in such a manner as to impair image quality. In addition, there is a need for heat development apparatuses that enable automatic feeding of migration imaging members into the apparatus and enable practical implementation in a machine environment. Further, there is a need for heat development apparatuses that enable automatic feeding of migration imaging members and enable uniform heat development. There is also a need for heat development apparatuses with a simple and compact design and which can easily accommodate imaging members of different sizes.